Secondary batteries such as lithium ion secondary batteries have characteristics such as compact size, light weight, high energy-density, and the ability to be repeatedly charged and discharged, and are used in a wide range of applications. Therefore, studies have been carried out in recent years with the objective of further raising the performance of secondary batteries through improvement of electrodes and other battery components.
An electrode for a secondary battery, such as a lithium ion secondary battery, normally includes a current collector and an electrode mixed material layer formed on the current collector. The electrode mixed material layer is formed, for example, by applying a slurry composition onto the current collector and then drying the slurry composition. The slurry composition contains a dispersion medium and, dispersed therein, an electrode active material, a binder composition containing a binder, and so forth.
In order to further improve the performance of secondary batteries, attempts have been made in recent years to further improve binder compositions used in electrode mixed material layer formation. In one specific example, it has been proposed that that binding capacity of an electrode active material to itself or between an electrode active material and a current collector can be increased and performance of a secondary battery can be improved by using a binder composition that contains two types of particulate polymers as a binder.
More specifically, PTL 1, for example, proposes a technique for increasing binding capacity of an electrode active material to itself or between an electrode active material and a current collector while also improving cycle characteristics of a secondary battery by using, as a binder, first rubbery resin particles composed of styrene-butadiene rubber particles having an average particle diameter of at least 130 nm and second rubbery resin particles composed of nitrile rubber particles having an average particle diameter of less than 130 nm.
Moreover, PTL 2, for example, proposes a technique for increasing binding capacity between an electrode active material and a current collector while also improving pressing processability of an electrode for a secondary battery by using, as a binder, (a) a polymer latex having a number average particle diameter of from 80 nm to 120 nm, a glass transition temperature of from 5° C. to 50° C., and a toluene gel content of at least 70% and (b) a polymer latex having a number average particle diameter of from 150 nm to 280 nm, a glass transition temperature of from −50° C. to 0° C., and a toluene gel content of at least 70%.